There are a number of industrial processes wherein a liquid and a gas are brought into direct contact with each other for effecting a transfer of heat and mass from one fluid to the other. The efficiency with which this direct contact heat-mass transfer occurs is primarily dependent on the amount of liquid surface area that is generated within the apparatus and comes into contact with the gas. Most of the apparatus specifically designed for this type of process employ some physical means, commonly called a heat-mass transfer media or fill assembly, whose primary purpose is to promote the generation of liquid surface area. Closely associated with this is the ability of the media to uniformly distribute the two fluids on the media surfaces and/or throughout the media section, and to assure they intimately mix while increasing fluid contact time as much as possible. This is accomplished by either promoting the generation of liquid droplets by means of a splash bar type heat-mass transfer media or by promoting the generation of thin liquid films on the surface of a cellular structure, commonly called a film type heat-mass transfer media. Clearly, this distinction relates only to the primary means of creating liquid surface area since splash type media will generate some liquid films and visa versa. The ability of the heat-mass transfer media to promote liquid-gas surface contact area and to promote intimate interaction between the two fluids and the media itself is only one key factor in the overall efficiency and applicability of the media. Of equal importance is the overall resistance to gas flow through the media that is a main factor in determining the fan energy required to move the gas through the apparatus. A given media may exhibit excellent ability to generate transfer surface area and intimate mixing and interaction between the fluids. However, if at the same time the resistance to flow and energy losses within the gas as it flows through the media is high, it will require a prohibitively high expenditure to fan energy to move the gas through the apparatus. As a consequence, the overall advantages of the media will be severely impaired or lost in practical application or otherwise limited to applications involving only small quantities of the fluids to be acted upon. To avoid or overcome this limitation, the method by which the media directs gas flow through the media must offer an aerodynamically efficient pathway for the gas thereby minimizing resistance to flow and energy losses within the gas flow due to excessive turbulence, etc. To accomplish this, the media design must minimize the projected area in a plane perpendicular to the gas flow direction and avoid surface configurations or the creation of gas flow regimes that result in restricted flow or excessive turbulence to avoid excessive energy losses within the gas. Thus, the flow of gas through the media must be aerodynamically efficient if the overall performance of the heat-mass transfer media is to be commercially successful.
In either type of heat-mass transfer media, both the liquid and gas are in a constantly changing dynamic state and individual and distinct elements of each fluid interact with adjacent elements of their own kind, elements of the other fluid and with the heat-mass transfer media in a complex way. A researcher may be able to isolate and analyze certain fundamental elements in this complex process and flow behavior and thereby gain a scientifically measurable understanding of what is important. However, there is no known precise way of evaluating the overall complex interactive behavior that actually occurs between fluids and their interaction with the heat-mass transfer media, and to predict performance superiority for a given media geometry as compared to others.
Further, it is never obvious or possible to combine known features of prior art designs to obtain an analytically predictable result. In this art, each heat transfer media design exhibits different performance characteristics which are dependent on the placement and geometry within a given design concept and method even though individual elements of the methods and means by which the design will function are clearly identifiable.
Examples of prior art splash type, counter current flow heat-mass transfer media generally describe simple, solid, rectilinear or trapezoidal shaped splash bar elements in horizontal, spaced apart relationship either assembled into grids or supported by wire hanger systems. In general, these designs exhibit significantly lower heat exchange efficiency as compared to film type media. Because of this, and higher energy costs, splash type counter current heat exchange media of the prior art have generally become obsolete.
Examples of film type media and assemblies are shown in U.S. Pat. No. 2,809,818, patented on Oct. 15, 1957 and a number of others both before and since this patent. Most of the film-type heat transfer media are composed of a plurality of thin, corrugated and specifically formed sheets. Assembled, adjacent sheets form cellular passageways where the gas and liquid may flow in counter current relationship to one another. In most configurations, the liquid flows as thin films adhering to the sheets and the gas flows uniformly filling the passageway. U.S. Pat. No. 3,262,682 illustrates one of the more effective film-type heat transfer media. All sheets are oriented and connected such that the corrugations extend at an oblique angle relative to a horizontal plane with every second layer having its corrugations oriented obliquely in the opposite direction. U.S. Pat. No. 4,950,430 is a similar, but more recent film type fill assembly example wherein the corrugated sheets are perforate. The perforations of this invention are taught as a means to generate thinner liquid films on the media surfaces and also as a means to further promote uniformity in the distribution of both liquid and gas. The corrugated sheets with the "special structures or surface treatments" which are therein described as the various perforate designs and features, are included as a primary means to enlarge and maintain the liquid contact area and to enhance the uniformity of the gas distribution. The perforations as taught in this invention act as liquid dividers, diverting the thin film liquid flow on a packing sheet around the perforations thereby aiding the horizontal and lateral spread of the liquid films. Said perforations are also taught as a means it enable liquid film flow from one side of the sheet to the other and also as a means to enable gas flow from one passage to another. In regards to gas flow through the perforations, it is important to note that in this media structure and method gas flow through the perforations will only occur if there is an inadvertent imbalance is gas flow between adjacent gas flow passages which are all of uniform size, and shape and thereby of themselves generally promote uniformity in gas flow and direction.
U.S. Pat. No. 5,316,626 also describes a film type heat-mass transfer media in the "Type I compartments." In this device all of the liquid is in film flow on perforate plate surfaces and all of the gas or vapor is forced through the perforations in the plates which are either flat or corrugated and may be placed either vertically or at an inclined angle. A more extreme example with almost identical method and operating means is found in U.S. Pat. No. 885,230 which forces all of the gas and a large portion of the liquid film on the plates to flow together through the plate perforations. Both of these prior art teachings result in extremely high resistance to gas flow and correspondingly high gas side energy losses limiting their application to very low mass flow rate processes.
U.S. Pat. No. 2,003,271 teaches a media consisting of vertically arranged members of perforate sheet metal in horizontal spaced apart relationship which are inclined to themselves with respect to the perpendicular, and to this end, may be buckled or corrugated in shape. These vertically arranged members are superposed in layers at an angle to one another and thereby create a "pervious mass" for continuously retarding and subdividing water flowing downwardly through the mass. While this arrangement provides for intimate mixing of liquid and gas, there is also a very high resistance to gas flow, thereby limiting this art to low flow applications. Further this patent relies almost exclusively on the shearing action of the sharp edges on the apertures formed by expansion of the sheet metal to subdivide the liquid and does not provide a means to extend the splash surface areas.
There are other Patents such as U.S. Pat. No. 1,549,068 and U.S. Pat. No. 3,804,389 which employ spaced apart wave forms assembled into vertically spaced apart imperforate grids where the heat-mass transfer media are designed to generate liquid surface by both splash and liquid film method in varying degrees. The emphasis in both of these patents is placed on enhancing liquid film surface development by employing simple waveforms in the grid structure and neither teaches the use of perforations for any purpose.
Among the problems associated with all of the film type media described or referred to above is their relatively higher resistance to gas flow. Even more importantly, recent experience has shown that these cellular film type designs, particularly in cooling tower applications, will plug up with dust and biological matter that are extracted from the atmospheric air and deposited on the surfaces of the passageways over time. Once this happens, film type designs become inoperable and replacement is the only practical solution. The gentle flow of liquid films in the passageways is insufficient to wash away these deposits. By contrast, splash type designs are self-cleaning as the vigorous splashing action of free falling liquid easily removes any sediment or deposits.
There are many splash bar type methods and apparatus for promoting heat-mass transfer in a direct contact heat exchange apparatus design for cross current flow relationship including U.S. Pat. No. 4,578,227 patented Mar. 25, 1986, and more recently U.S. Pat. No. 5,112,537 as well as other prior art examples referred to in these patents. Almost without exception, these devices are specifically designed for cross current flow relationship between the fluids and if applied to counter current fluid flow relationship, will result in very high gas energy losses since they all present a broad projected surface area perpendicular to the direction of gas flow. This prohibits their effective usage in counter current flow applications. The single exception is U.S. Pat. No. 3,758,088 that, if applied in the counter current liquid-gas flow relationship, would provide a fairly aerodynamically efficient gas flow regime. However, this patent teaches splash type fill assembly consisting of the splash bars wherein the members have nonplanar upwardly facing splash surface. Said members are of transversely arcuate configuration defining at least a portion of a sine wave. This configuration limits the project splash surface by the very nature of the splash bar shape. Any attempt to increase projected splash surface area in the direction of liquid flow by simply increasing the sine wave amplitude would constitute an increase in splash surface area but at the same time will increase gas flow turbulence and result in excessive energy losses in the gas flow. Further, this patent does not teach the incorporation of perforate surfaces in any form.
While the differences in splash bar designs and the various uses of perforate surfaces as found in prior art designs may appear subtle, those skilled in the art will recognize that geometric shape and relative positioning are highly significant in terms of their impact on splash effectiveness, liquid distribution and fragmentation and the gas flow regime, distribution and energy losses. These differences are magnified by the fact that a typical media assembly area contains a very large number of individual splash bar elements, each of which influences the liquid and gas dynamics and the performance of it's neighbors. Clearly much is yet to be done to obtain the ultimate functional relationship between gas and liquid in a counter current splash bar design and assembly matrix.